Fuel cells continue to show great commercial promise throughout the world as an alternative to conventional energy sources. This commercial promise should continue to grow as energy shortages become more acute, environmental regulations become more stringent, and new fuel cell applications emerge. See "FUEL CELLS", Encyclopedia of Chemical Technology, 4th Ed., Vol. 11, pp. 1098-1121.
Despite improvements in fuel cell technology, however, long felt needs exist to increase power output, reduce initial cost, improve water management, and lengthen operational lifetime. Initial cost reduction can be most easily achieved by reducing the precious metal content of the fuel cell electrode. Such reduction, however, generally results in power output loss which blocks commercialization efforts.
There are different types of fuel cells, but they each produce electrical energy by means of chemical reaction. One type of increasing import, the "polymer electrolyte membrane fuel cell" (PEMFC), comprises a membrane electrode assembly (MEA) typically made of an ionically conducting polymeric membrane sandwiched between two electronically conducting electrodes. For commercial application, multiple MEAs can be electronically connected to form a fuel cell stack (i.e., "stacked"). Other components associated with typical PEMFCs include gas diffusion media and current collectors, the latter of which can also serve as bipolar separators and flow field elements. PEMFCs have been reviewed in the literature. See S. Srinivasan et al.; J. Power Sources; 29 (1990); pp. 367-387.
In a typical PEMFC, a fuel such as hydrogen gas is electrocatalytically oxidized at one electrode (anode). At a the other electrode (cathode), an oxidizer such as oxygen gas is electrocatalytically reduced. The net reaction results in generation of electromotive force. Elevated temperature can accelerate this reaction, although one increasingly important advantage of the PEMFC is that lower temperatures (e.g., 80.degree. C.) can be used. The fuel cell reactions are generally catalyzed by precious transition metal, commonly a noble metal such as platinum, which is present in both anode and cathode. Because the fuel cell is often operated with use of gaseous reactants, typical electrodes are porous materials (more generally, reactant diffusive materials) having the catalytically active metal at the porous surfaces. The metal can be in different morphological forms, but often it is in particulate or dispersed form and supported on carbon. Fuel cell performance may depend on the form of catalyst. See Poirier et al.; J. Electrochemical Society, vol. 141, no. 2, February 1994, pp. 425-430.
Fuel cell systems are complex because the reaction is believed localized at a three-phase boundary between ionically conducting membrane, gas, and carbon supported catalyst. Because of this localization, addition of ionically conductive material to the electrode can result in better utilization of catalyst as well as improved interfacial contact with the membrane. However, the additional ionic conductor can introduce extra cost, especially when perfluorinated conductors are used, and can increase the complexity of electrolyte water management, all important to commercialization.
One general approach to minimize loading of expensive catalytic metal has been to use smaller catalyst particles. However, long operational lifetimes are particularly difficult to achieve with low catalyst loadings. Also, catalyst particle size may be unstable and increase by agglomeration or sintering.
Another approach has been to concentrate the metal at the membrane-electrode interface. See Ticianelli et al.; Journal of Electroanalytical Chemistry and Interfacial Electrochemistry; Vol. 251 No. 2, Sep. 23, 1988, pp. 275-295. For example, 500 angstrom dense layers of metal catalyst reportedly have been sputtered onto certain gas diffusion electrodes before sandwiching the ionically conducting membrane between the electrodes. Apparently, however, sputtered layers thinner than 500 angstroms have not been reported, possibly because of the difficulty in making uniform thinner layers. Moreover, other types of electrodes and deposition techniques may not be suitable, water balance may be upset, and testing often is not carried out under commercial conditions. In sum, it is recognized that mere depositing a thin layer of catalyst onto the electrode does not guarantee a suitable MEA. According to the Srinivasan article noted above, sputtering may not be economically feasible compared with wet chemical deposition methods. Thus, in general, industry has not accepted this approach as realistic.
Additional technology is described in the patent literature including, for example, U.S. Pat. Nos. 3,274,029; 3,492,163; 3,615,948; 3,730,774; 4,160,856; 4,547,437; 4,686,158; 4,738,904; 4,826,741; 4,876,115; 4,937,152; 5,151,334; 5,208,112; 5,234,777; 5,338,430; 5,340,665; 5,500,292; 5,509,189; 5,624,718; 5,686,199; and 5,795,672. In addition, deposition technology is described in, for example, U.S. Pat. Nos. 4,931,152; 5,068,126; 5,192,523; and 5,296,274.